It is recalled that the diffusion-welding technique consists in putting two plates of a given material into contact at high temperature under a certain amount of pressure and for a certain length of time. The two plates then become welded together by atomic diffusion, thereby presenting the advantage of forming a bond structure that is equivalent to the base structure of the material.
When diffusion welding is associated with superplastic forming, an anti-diffusion or “stop-off” substance is used to prevent diffusion welding taking place in those zones of the facing faces of the plates that are subsequently to be inflated in order to obtain a hollow mechanical part.
Thus, the anti-diffusion substance forming a diffusion barrier (also known as a stop-off substance) is applied in predefined zones on at least one of the facing faces of the plates of superplastic material so that when the diffusion-welding step has finished the plates are not welded together in the zones covered in the anti-diffusion substance, which substance generally comprises a filler of refractory material that inhibits diffusion of the atoms of the plates that are to be welded together.
The assembly of plates that have been selectively welded together by diffusion welding is generally subsequently subjected to superplastic forming by heating the assembly to a temperature that is compatible with superplastic behavior of the plate material, in a mold that is generally closed. An inert gas is then injected under controlled pressure into the non-welded zones of the assembly, thus enabling the plates to be inflated to match the profile of the mold.
Naturally, the quality of the welding that results from the diffusion-welding step depends on operating parameters: temperature, pressure, and time, and also on parameters associated with the elements that are to be assembled together: metallurgical structure, surface state (cleanness, roughness). Consequently, it is essential to eliminate all sources of contamination on the surfaces to be assembled together prior to raising the temperature in the diffusion-welding step.
This cleaning of the surfaces is conventionally performed by creating a vacuum in the cavity formed by the two surfaces to be welded together. However, when diffusion welding is associated with superplastic forming, an anti-diffusion substance is used, generally comprising an organic binder together with a powder of an anti-diffusion material constituted by a filler of refractory material such as a ceramic (e.g. yttrium oxide, alumina, or boron nitride) or graphite. This anti-diffusion material (or stop off product) inhibits the diffusion of atoms from the material of the plates to be welded together.
After applying the anti-diffusion substance in a predefined pattern corresponding to the zones of the surfaces that are not to be joined together by diffusion welding, the binder is generally degraded so as to retain only the powdered anti-diffusion substance, presenting the anti-diffusion properties.
This application of the anti-diffusion substance is generally performed by the so-called “silkscreen printing” technique which makes use of printing screens each comprising a frame surrounding a mesh through which the fluid to be deposited is caused to pass in a predefined pattern. The mesh is implemented in the form of a stretched web (or cloth) of woven yarn and presents sealed portions that prevent fluid from passing through in zones that are not to be coated in anti-diffusion substance.
That technique presents resolution which depends in particular on the size of the mesh and the diameter of the yarn, where the size of the mesh must be large enough to allow the fluid for deposition to pass through but small enough to avoid jagged-edge phenomena. In addition, the silkscreen printing technique requires a system for positioning the frame relative to the part and for performing multiple adjustments (web tension, distance between the web and the part, . . . ), the silkscreen web suffering wear over time which is revealed by distortion that leads to the deposited patterns shifting.
It will be understood that that technique is relatively awkward to implement, that it gives a result that is not strictly reproducible over time, and that it requires very good control over the viscosity of the anti-diffusion substance.
It is also known to deposit the anti-diffusion substance by spraying through mask, as described in document FR 2 739 045 which relates to a method of manufacturing a hollow blade for a turbomachine. That spraying technique presents the following steps:
a) applying an organic type mask to at least one face of at least one of the primary parts;
b) cutting through the mask with a predefined pattern representing the boundary between the welded and non-welded zones, using a special tool under computer control;
c) peeling off the mask in the zones that are not for welding;
d) cleaning the surfaces;
e) depositing an anti-diffusion substance on the previously prepared surfaces;
f) peeling off the remainder of the mask;
g) pre-baking treatment of the anti-diffusion substance;
h) cleaning and inspecting the surfaces to be welded.
It is observed that step c) of peeling the mask away from the zones that are not to be welded leads to small local tears in the mask, thereby degrading straightness at the edge of the deposit. In addition, the peeling of step f) leads to local zones at the periphery of the deposit being torn away and/or collapsing. These two phenomena therefore contribute to reducing the geometrical accuracy of the deposit, and consequently the quality of the surfaces to be welded. In addition, it is important to control the viscosity of the anti-diffusion substance so that during step e) it covers the previously prepared surfaces properly.
It will thus be understood that that spraying technique is awkward to implement, requires a long time to perform, and leads to a result with imperfections.
Finally, document EP 0 849 029 proposes applying the anti-diffusion substance by direct deposition performed using the fluid jet method. That technique is close to the ink jet method of printing since it uses a print head that moves over the plate under the control of a computer, with fluid being transferred by jets from a tank onto the face of the plate in a predefined pattern.
That technique thus enables the substance to be deposited directly on the part without making use of a silkscreen or a prior step of depositing a mask, thereby simplifying the operation of deposition, and eliminating the step of making, maintaining, and adjusting/inspecting silkscreen frames.
Nevertheless, that direct deposition technique using the fluid jet method requires the viscosity of the fluid forming the anti-diffusion substance to be adjusted accurately, in particular by adding anti-settling and anti-coagulation agents. These added elements generally cause the deposited substance to be contaminating to the surface on which it is deposited, particularly when said surface is made of a titanium-based alloy.